Strict Mode

JavaScript · Reference cheat sheet

📋 Overview

Strict mode ("use strict") opts into a cleaner, safer JavaScript subset: fewer silent failures, no accidental globals, and this is undefined in bare calls. ES modules and class bodies are strict by default.

🔧 Core concepts

• Enable: "use strict"; at top of script or function body.
• Modules / classes: always strict.
• No accidental globals: assigning undeclared vars throws.
• this: undefined in non-method calls (not global object).
• Forbidden: with, octal literals like 0123, deleting plain names, duplicate params (legacy).
• arguments: no dynamic link to parameters; arguments.callee forbidden.

"use strict";
function f() {
  return this; // undefined when called as f()
}

💡 Examples

"use strict";

// ReferenceError — would create global in sloppy mode
// x = 1;

let x = 1;
// delete x; // SyntaxError in strict

function sum(a, a) {
  // SyntaxError: duplicate params in strict
}

function show() {
  console.log(this);
}
show(); // undefined (strict) vs window (sloppy browser)

// Silent failure becomes throw
const obj = Object.freeze({ a: 1 });
// obj.a = 2; // TypeError in strict

// Octal
// const n = 012; // SyntaxError — use 0o12

// eval/arguments as binding names illegal
// function eval() {} // SyntaxError

// Function-level strict
function legacyCompat() {
  // sloppy if script is sloppy
}
function modern() {
  "use strict";
  // strict only inside modern
}

⚠️ Pitfalls

• Concatenating scripts can lose a leading "use strict" if not first statement.
• Libraries expecting sloppy this === window break under strict — bind explicitly.
• Strict is per-function/script — mixed modes in one bundle confuse debugging.
• Don’t rely on sloppy features; write as if everything is strict (modules already are).

🔗 Related

• this_keyword.md — undefined this
• import_export.md — modules are strict
• error.md — thrown errors
• freeze_seal.md — TypeError on mutate
• equality.md — cleaner semantics